Abstract
Thermoelectric materials and devices are regarded as the important energy materials have been the subject of intensive study, primarily because of their abilities of direct transformation between heat and electricity. Recently, due to the development of advanced semiconductor processes and nanotechnology, the researches of thermoelectric materials are moving into new territory. Lead telluride (PbTe) and Cobalt triantimonide (CoSb3) alloys are the most widely used mid-temperature thermoelectric materials. Additionally, the Sb-Se-Pb-Sn alloys have been recently examined among various promising thermoelectric materials. In Sb-Se-Pb-Sn system, the PbSe, Sb2Se3 and SnSe compounds are of interests to thermoelectric applications because of their outstanding performances reported by numerous groups. Soldering plays an important role of assembly of thermoelectric devices. Sn-Pb alloys are the preferred jointing material of electronic industry owning to its attractive price and relative low melting point. Nevertheless, there are healthy and environmental issues associated with the toxicity of Pb containing solders, the researches of Pb-free solders were taken seriously. In a variety of Pb-free solder systems, Sn-Ag-Cu alloys are used widely. Besides, Sn-Ag-In, Sn-Ag-Zn, and Sn-In-Zn solder alloys are also promising Pb-free solders. Additionally, there are studies about using Sn-Ag-In-Zn quaternary alloys as electronic solders. Construction of Sn-Ag-In-Zn system is fundamentally important for designing the solder alloys with better mechanical properties and higher stability. However, the phase diagrams and thermodynamic models of Ag-In-Zn ternary system have not been constructed. Knowledge of phase equilibria is fundamentally important for materials development and applications. Phase diagrams can be obtained by experimental determinations and calculations. The CALPHAD (Calculation of Phase Diagram) approach is the most popular calculation method. Based on the information of the thermodynamic properties and phase equilibria of system, the assessment of the thermodynamic models can be determined. In this study, the phase diagrams of Ag-In-Zn system of Sn-Ag-In-Zn quaternary system and Pb-Sb-Se, and Sn-Sb-Se ternary systems of Sb-Se-Pb-Sn quaternary systems are determined. The microstructures, compositions, and diffraction peaks were determined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), electron probe X-ray microanalyzer (EPMA) and X-ray diffractometer (XRD). A differential thermal analysis (DTA) was used to determine the reaction temperatures of the invariant reactions. The CALPHAD approach and AIMD (Ab-initio molecular dynamics) have been used for the assessment of the thermodynamic models of Sb-Se and Sn-Se binary systems. The liquidus projection and isothermal section of Ag-In-Zn lead-free solder system and Pb-Sb-Se, Sn-Sb-Se thermoelectric systems are experimentally determined. There are 10 primary solidification phases: (Ag), (In), (Zn), Beta-(Ag3In), Zeta-(Ag3In), Gamma-(Ag9In4), AgIn2, Beta-(AgZn), Gamma-(Ag5Zn8), and Epsilon-(AgZn3) in the Ag-In-Zn ternary system. Moreover, there are five ternary invariant reactions: Liquid=(In)+AgIn2+Gamma-(Ag5Zn8), Liquid=Epsilon-(AgZn3)+(In)+(Zn), Liquid+Beta-(AgZn)=Gamma-(Ag5Zn8)+Gamma-(Ag9In4), Liquid+Zeta-(Ag3In)=Gamma-(Ag5In8)Beta-(AgZn) and Liquid+(Ag)+Beta-(AgZn)=Zeta-(Ag3In). In the isothermal section at 500oC, there is no ternary compound and there are 6 tie triangles : (Ag)+Zeta-(Ag3In)+Beta-(AgZn), Zeta-(Ag3In)+Gamma-(Ag9In4)+Beta-(AgZn), Zeta-(Ag3In)+Gamma-(Ag9In4)+Liquid, Gamma-(Ag9In4)+Beta-(AgZn)+Liquid, Beta-(AgZn)+Gamma-(Ag5Zn8)+Liquid and Gamma-(Ag5Zn8)+Epsilon-(AgZn3) in the 500oC isothermal section of Ag-In-Zn ternary system. There are 8 primary solidification phases, which are (Pb), (Sb), (Se), PbSe, Sb2Se3, Pb6Sb6Se17, PbSb2Se4 and Pb2Sb9Se9 phases in the liquidus projection of Pb-Sb-Se ternary system. Among of all the primary solidification phases, the ternary compound, Pb2Sb9Se9 is the new ternary compound which has not been reported before. The XRD pattern of Pb2Sb9Se9 is determined by X-ray diffraction analysis. Furthermore, Five invariant reactions: Liquid=PbSe+(Sb)+(Pb), Liquid=Pb2Sb9Se9+PbSb2Se4+Sb2Se3, Liquid+(Sb)=Pb2Sb9Se9+Sb2Se3, Liquid +PbSe =PbSb2Se4+Pb2Sb9Se9, Liquid+PbSe+(Sb)=Pb2Sb9Se9 are determined by thermal analysis. There are 9 tie-triangles, which are PbSe+(Sb)+Liquid, PbSe+PbSb2Se4+(Sb), PbSb2Se4+Pb2Sb9Se9+(Sb), PbSb2Se4+Pb2Sb9Se9+Sb2Se3, Pb2Sb9Se9+Sb2Se3+(Sb), PbSe+Pb6Sb6Se17+PbSb2Se4, PbSe+Pb6Sb6Se17+Liquid, Pb6Sb6Se17+PbSb2Se4+Sb2Se3 and Pb6Sb6Se17+Sb2Se3+Liquid, in the 400oC isothermal section of Pb-Sb-Se ternary system. There are 10 primary solidification phases, which are (Sn), (Sb), (Se), Sb2Sn3, SbSn, SnSe, SnSe2, Sb2Se3, Sn2Sb9Se9 and SnSb2Se4 phases in the liquidus projection of Sn-Sb-Se ternary system. It is worthy of mentioning that Sn2Sb9Se9 ternary phase is another new compound which has not been reported in the literature. The XRD pattern of Sn2Sb9Se9 is determined by X-ray diffraction analysis. Seven invariant reactions: Liquid+Sb2Sn3=SnSe+(Sn), Liquid+SbSn=SnSe+Sb2Sn3, Liquid+(Sb)=SbSn+SnSe, Liquid+Sn2Sb9Se9=SnSb2Se4+Sb2Se3, Liquid+Sn2Sb9Se9+(Sb)=Sb2Se3, Liquid+(Sb)+SnSe=Sn2Sb9Se9 and Liquid+Sn2Sb9Se9+SnSe=SnSb2Se4 are experimentally determined in this study. There are 9 tie-triangles, which are Liquid+SbSn+SnSe, SbSn+SnSe+(Sb), SnSe+(Sb)+Sn2Sb9Se9, (Sb)+Sb2Se3+Sn2Sb9Se9, SnSe+Sn2Sb9Se9+SnSb2Se4, Sb2Se3+Sn2Sb9Se9+SnSb2Se4, SnSe+SnSe2+SnSb2Se4, SnSe2+SnSb2Se4+Sb2Se3 and SnSe2+Sb2Se3+Liquid, in the 400oC isothermal section of Sn-Sb-Se ternary system. In the calculation of Sb-Se binary system, the functions of Gibbs energies of pure elements are taken from SGTE database, and the associate model is used to describe the liquid phase. The new thermodynamic description of the Sb–Se system is proposed, and good agreement between available literature data and calculated results is found. The re-optimized thermodynamic model of Sb-Se binary system may allow the construction of calculations upon multicomponent. Another important investigation in this study is examining the existence of miscibility gap of Sn-Se binary system by using AIMD simulations. The enthalpy of mixing in liquid phase is calculated, and the results are fitted with Redlich-Kister polynomial for obtaining the new parameters used in CALPHAD approach.